I think one criteria should be that the widest part of your foot matches the widest part of the shoe. This is a bit hard to judge accurately, but it should give you a rough idea.
With the boots unzipped, how far can you mash your foot forward? This will give you some idea as to the toe room. If the toes narrow quite a bit, then the width will prevent your foot from going too far forward. I usually find if I push my foot forward in the shoe, I have about one finger's room behind my heel. This is again, just a rough guide.
Probably the best thing to do is just to try on a smaller size and see how it fits.
With the boots unzipped, how far can you mash your foot forward? This will give you some idea as to the toe room. If the toes narrow quite a bit, then the width will prevent your foot from going too far forward. I usually find if I push my foot forward in the shoe, I have about one finger's room behind my heel. This is again, just a rough guide.
Probably the best thing to do is just to try on a smaller size and see how it fits.
